This invention relates to an automotive vehicle with an internal combustion engine having a variable cam timing unit (VCT), and in particular to an arrangement of a pressurized fluid supply for a VCT of an automotive vehicle internal combustion engine.
Automotive vehicle engines with reciprocal pistons typically have a plurality of cylinder combustion chambers with the reciprocating pistons mounted therein. Each piston is pivotally connected with a piston rod, which is pivotally connected with a crankshaft. A timing gear is mounted at an end of the crankshaft. Typically, each cylinder has at least one intake valve and one exhaust valve. Both the intake valve and the exhaust valve are spring-loaded to a closed position. Each intake and exhaust valve is associated with a rocker arm. To operate the valves, the rocker arms are moved by a set of contacting cam lobes. The cam lobes are mounted on an elongated member known as a camshaft. Attached at an extreme end of the camshaft is a camshaft pulley. The camshaft pulley is powered by the crankshaft via a timing chain or belt which is looped over the camshaft pulley and a crankshaft timing gear. Accordingly, the camshaft is synchronized with the crankshaft and the timing of the opening and closing of the intake and exhaust valves is fixed with respect to the position of the piston within the cylinder combustion chamber.
In an effort to improve the environment by decreasing polluting emissions and increasing vehicle gas mileage, it has become desirable to allow the timing of the cylinder valve operation to vary with respect to the piston position within the cylinder chamber. To provide for the variable valve timing operation, a VCT is provided on the camshaft.
An example of a VCT is a dual oil feed vane-type VCT. A dual oil feed vane-type VCT provides an inner member or hub that is fixably connected to an end face of a camshaft. The hub has a series of vanes which are captured in cavities or pressure chambers provided in an outer member which is concentrically mounted on the hub. The outer member incorporates the camshaft timing pulley. The vanes circumferentially bifurcate the pressure chambers into an advance side and a retard side. A spool valve, fluidly communicative with the pressure chambers via the inner member and the camshaft, controls the fluid pressure in the advance side and retard side of the pressure chambers. Accordingly, the angular position of the timing pulley versus the crankshaft can be varied by controlling the fluid in the advance and retard pressure chambers.
Another example of a dual oil feed VCT is a helical gear type VCT. The helical gear type VCT has an outer member attached to an inner member or hub along a helical gear connection. A pressure chamber is provided between the inner and outer members. The pressure chamber is axially bifurcated by a pressure boundary which contacts the outer member to move the same with respect to the inner member. The outer member can axially move with respect to the inner member. The helical gear interconnection between the inner and outer members causes the outer member to rotate with respect to the inner member and accordingly changes angular position with respect to the inner member.
Both of the aforementioned VCTs utilize engine lubricating oil pressure and flow to phase the camshaft. The VCT must meet minimum phase speed requirements to achieve the desired fuel economy and emission benefits as well as acceptable drivability and the avoidance of stumble/stall conditions. Typically, the engine oil pump in most vehicles cannot meet the oil pressure instantaneous flow requirements of a VCT, especially at low engine speeds and high oil temperatures. Accordingly, most engine oil pumps need to be upsized to a considerable higher flow rate. The increase in flow rate brings a disadvantage of parasitic losses and fuel economy degradation across the entire engine speed range. Since the oil pump pressure requirements are most difficult to meet at low engine speeds and high oil temperatures, a major issue exists with low (500 rpm) idle speeds and ever lower lug limits (as low as 900 rpm with a continuously variable transmission). At these conditions, the prior art approach of increasing the oil pump size will not meet the VCT unit demands, particularly with an engine having dual independent valve systems which require four VCT units for a V-type engine (one inlet valve camshaft, one outlet valve camshaft for each engine bank). It is desirable to provide an oil supply arrangement for an engine with VCT units which do not require a significantly larger engine oil pump than what is presently required.
Scheidt et al. U.S. Pat. No. 5,915,348 provides an oil supply arrangement for a VCT that supplies pressurized oil to the VCT when the motor is off to remove gas pockets which can occur. However, Scheidt et al. does not reveal an arrangement to meet the oil pump pressure requirements at low engine speed and high oil temperatures.